Best way to create a portal site with multiple subdomains?

I recently moved my one topic site to another domain. The domain is more general and allows me to write about more things instead of being pigeon holed into one topic.

So I moved my old site from http://www.oldsite.com to http://topic.newsite.com.

What I would like to do is allow each subdomain to be about one topic while the top domain would be a portal to show everything from all domains. Basically I want the subdomain extensions to not access anything from another subdomain but let all the content from the subdomains to "bleed" into the portal.

Is there a simple and relatively maintenance free way to do this?

I thought that I could be to use RSS to gather up all the content from the domains and place them on the portal. But I am looking for something maybe a little less "hackish" and maybe more flexible in the long run.

Thanks!